Selectively programmable variable-voltage direct-current voltage source

ABSTRACT

A continuous cardiac output monitor includes a general-purpose monitoring console with local display and communication facilities, and a module removably interfacing with the console to configure the latter for performing continuous cardiac output monitoring. The module includes a switch-mode high efficiency power amplifier for providing electrical heating power at a selected voltage, frequency, and wave form to a heating element of a continuous cardiac output monitoring catheter, which catheter at a distal end portion thereof is immersed in the blood flow of a patient. The catheter effects a temperature transient in the patient&#39;s blood flow by the controlled application of electrical resistance heating utilizing electrical power from the power amplifier, and this temperature transient is sensed and used to derive a value for the patient&#39;s cardiac output.

This is a division of application Ser. No. 08/268,217 filed on Jun. 29,1994 now U.S. Pat. No. 5,636,638.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power amplifiers or electrical powersupplies. More particularly, the present invention is in the field ofpower amplifiers for supplying electrical power at a chosen frequency orfrequencies and at a selected and variable voltage level or levels.Still more particularly, the present invention is in the field ofregulated electronic power amplifiers or supplies which supply aselected level of electrical power at a controlled frequency andvariably controlled voltage to an electrical load. With particularity tothe application environment of such power amplifiers or power supplies,the present invention relates to such a power amplifier which isparticularly configured and constructed for use in the medicalenvironment to supply resistive heating power to a continuous cardiacoutput monitoring catheter. Thus, the present invention also is in thefield of apparatus and method for monitoring the cardiac output of ahuman patient.

2. Related Technology

Conventionally, cardiac output monitoring for patients experiencing acardiac crisis, such as may occur over a period of time following acoronary occlusion, is to periodically inject a quantity (or bolus) ofchilled saline solution into the patient's circulatory system at aselected location. A temperature monitoring catheter is used at anotherselected location to sense the temperature-versus-time relationship ofthe blood flow so that a value of cardiac output can be derived. Thistechnique is known as thermodilution, and provides a good signal tonoise ratio of the pulmonary blood flow as it is cooled by the chilledsaline solution as compared to the normal temperature of blood flow inthe pulmonary artery prior to and after the injection of the bolus ofsaline. A relationship known as the Stewart-Hamilton equation is used toderive the cardiac output value.

Unfortunately, this conventional technique is dependent on the skill ofthe person who performs the saline injection. That is, the rate anduniformity over time with which the bolus of saline solution is injectedcan influence the accuracy of the result. Consequently, a number of suchtests over a period of time are used to determine an average value ofcardiac output. Detection of a trend or long-term change (over a periodof hours, for example) in cardiac output is very difficult with thisconventional technique. Also, the injection of chilled saline may havethe disadvantage for some patients of adding a relatively large quantityof water to the blood stream. This water must be removed by thepatient's kidneys.

Another conventional cardiac output monitoring technique utilizes acatheter instilled through the right atrium and right ventricle of theheart, and from the heart into the pulmonary artery. A resistanceheating element externally carried by this catheter is used tointermittently slightly heat the pulmonary blood flow from the heart asthis blood flows toward the patient's lungs. Downstream of the heatingelement, the catheter carries a temperature sensing element. Thetemperature-versus-time relationship of the sensed blood flow cansimilarly be used to derive a value for cardiac output. This techniquehas the advantage of providing substantially continuous monitoring ofcardiac output. However, the signal-to-noise ratio of the heated bloodtemperature in comparison to the normal body temperature of blood flowexisting prior to and after an interval of heating is very low. Thismust be the case because the blood cannot be heated excessively ordamage will result to formed blood cells. Consequently, techniques havebeen developed to heat the pulmonary blood flow on a pseudo-randombasis, so that the resulting temperature variations can be detected anddistinguished from the otherwise normal slight variations in temperatureof the pulmonary blood flow.

For reasons of patient safety and avoidance of electromagneticinterference with or effect upon other monitoring and treatmentapparatus which may also be in the medical environment around a patient,a frequency of 100 KHz has been recognized as the most desirable for usein powering the resistance heating element of the monitoring catheter.With this fixed frequency of applied power for the resistance heatingelement of the catheter, a variable voltage level is used to control thepower level of energy liberated at the heating element into thepatient's pulmonary blood flow. This control on the level of heatingenergy liberated into the patient's blood flow must be carefullycontrolled because the actual rate of blood flow circulation for thepatient may be decreased or impaired, so that overheating must beavoided.

In addition to the above, it is increasingly recognized that the modernmedical environment is restrictively complex. That is, the complexity ofmedical monitoring and treatment apparatus which must be used with someseriously ill or injured patients restricts access to the patient andpresents the risk of error or malfunction of the apparatus.Additionally, hospitals and clinics face a significant burden inmaintenance, service, storage, and logistical planning of theavailability of this complex and expensive medical apparatus. As aresult, an increasingly popular trend in the hospital, clinical, andportable medical treatment environments (fire departments, emergencymedical teams, and military portable field hospitals, for example) is touse a general purpose monitoring device which can be electronicallyconfigured to serve a variety of monitoring functions.

Configuration of the monitoring device is accomplished by simplyplugging into the console of the general purpose monitoring device oneor more modules containing the circuitry and stored informationnecessary to accomplish particular monitoring functions. In the hospitaland clinical environment, for example, this technology has the advantagethat the general purpose monitors may be installed in or left in thepatient rooms and in the emergency or critical care areas, for example.These monitors need not be moved about the hospital or clinic. Themonitors are simply configured to perform various monitoring functionsas are necessitated by the condition of the particular patient byplugging the appropriate modules into the monitor consoles. Only themodules need to be moved about the hospital or clinic. The modulesthemselves are comparatively small, light and inexpensive. Storage ofthe modules when they are not in use requires far less space than doesthe conventional monitoring equipment. Also, movement of the necessaryconfiguration modules about the hospital or clinic environment does notpresent nearly the burden for hospital staff as does the movements ofconventional monitors.

That is, conventional monitors are relatively large, heavy, andexpensive pieces of equipment, which are generally mounted on wheeledcarts. Each time a monitor is moved from one location to another withina hospital, for example, there is a certain risk that it will be damagedin the process of movement. Also, the physical movement of the monitorrequires the services of a relatively strong member of the hospitalservice personnel, for example, to move the wheeled cart and monitoronto and off of hospital elevators. On the other hand, the configurationmodules of modular type monitoring equipment are small enough to becarried by hand from one location to another. In fact, several of thesemodules can be carried at a time by one person if necessary. A singlewheeled cart of a size comparable to one conventional monitor can carryseveral to several dozen of the configuration modules for a modularmonitoring system.

With respect to the conventional continuous cardiac output monitors, themonitor includes a linear electronic power amplifier capable ofsupplying a variable power level and fixed frequency of electricalalternating current power output, and which provides electrical heatingpower to the resistance heating element of the continuous cardiac outputmonitoring catheter. This conventional linear power amplifier isphysically too large to be accommodated within the envelope of amonitoring module of the newer modular-type of monitoring apparatus.Also, the conventional power amplifier is of a power efficiency so lowthat although only about fifteen watts of power is dissipated into thepatient's blood flow on an intermittent basis, about thirty toforty-five watts, or more, of power is liberated as heat into theconsole of the conventional continuous cardiac output monitor. That is,the efficiency of these conventional linear power amplifiers may be aslow as 25 percent. Were this level of heat to be liberated within amonitoring module, assuming that the conventional linear power amplifiercould somehow be physically fitted into the module, the conventionalplastic casing of the module could be warped or melted by the resultinghigh temperatures.

SUMMARY OF THE INVENTION

In view of the deficiencies of the related technology as explainedabove, an object for the present invention is to provide a poweramplifier with a selectively programmable variable-voltagedirect-current voltage source which avoids one or more of thesedeficiencies.

More particularly, an object of the present invention is to provide apower amplifier for a continuous cardiac output monitoring apparatuswhich allows the power amplifier to be physically fitted within aconfiguring module for a modular-type of monitoring system.

Still another object for the present invention is to provide such apower amplifier having such a high level of efficiency that apermissibly small level of heat energy is liberated from the poweramplifier, allowing the power amplifier to be housed in a modulecompatible with conventional modular monitoring apparatus.

Particularly, the present invention relates to apparatus and method forelectrically heating cardiac blood flow within the heart of a humanpatient, and for sensing the temperature versus time relationship of theblood flow in the pulmonary artery. The power amplifier providesalternating current electrical power at a particular frequency chosenbecause of the particular safety of this frequency for the patient, andrelative freedom of this frequency from the production ofelectromagnetic interference which could affect other medical apparatusbeing used in the treatment of the patient. The alternating powersupplied is of variable voltage level to control the energy dissipatedin a resistive load from which heat energy is applied to cardiac bloodflow. The heat energy is intermittently according to a pseudo randomalgorithm to provide a temperature transient in the patient's cardiacpulmonary blood flow, which transient is sensed in order to derive avalue for cardiac output of the patient.

Accordingly, the present invention provides a selectively programmablevariable-voltage direct-current voltage source including a voltageregulator circuit and a programmable variable-resistance circuit. Anoutput voltage of the voltage regulator circuit is defined as a ratiofunction of two controlling resistors which are connected to the voltageregulator circuit. The programmable variable-resistance circuit includesplural resistors which are connected in parallel and which share acommon connection with the voltage regulator circuit. Each resistor hasa differing resistance value. The variable-resistance circuit alsoincludes switching means connected to a microprocessor and the pluralresistors. The switching means receives instructions from themicroprocessor and then responsively connects selected ones of theplural resistors in parallel to the voltage regulator circuit tocooperatively define one of the two controlling resistors.

According to one aspect of the selectively programmable variable-voltagedirect-current voltage source of the present invention is that theswitching means may include a plurality of digitally controlled analogswitches, each one of which has connection to respective ones of theplural resistors. When the analog switches are closed, respectiveresistors of the plurality of resistors are connected in parallelbetween the voltage regulator circuit and the common connection. Thevariable-resistance circuit may further include a digital microprocessorconnected to the analog switches for commanding selective closure ofparticular analog switches to alter the effective parallel resistancevalue connected to the voltage regulator circuit.

According to further aspects of the present invention, the voltageregulator circuit may include a Linear Technology LTC 1149 circuit, andthe plurality of digitally controlled analog switches may include eithera Siliconix 9956DY chip or a Harris DG412DY chip. The Siliconix 9956DYchip is preferably connected to resistors which have resistance valuesup to about 1.6 Kohm, and the Harris DG412DY chip is preferablyconnected to resistors which have resistance values of at least about1.6 Kohm.

According to another aspect of the selectively programmablevariable-voltage direct-current voltage source is that the voltageregulator circuit may be connected to the microprocessor. Thisconfiguration allows the voltage regulator circuit to be switched offdirectly by the microprocessor, thereby yielding an output voltage ofzero.

An advantage of the present invention resides in the freedom fromharmonic interference in the regulated power supplied by the poweramplifier. That is, such harmonic interference frequencies aresubstantially not present at all in the alternating current powersupplied by the present power amplifier. The present power amplifiersupplies essentially pure sine wave alternating current electricalpower. Further, the small size, light weight, low cost, frequencystability, fault tolerance (actually, redundant fault tolerance), andgood accuracy of the power level regulation provided by the presentpower amplifier are individually and in combination better than can beachieved with conventional power supplies.

Additional objects and advantages of the present invention will beapparent from a reading of the following description of a particularlypreferred exemplary embodiment of the invention taken in conjunctionwith the appended drawing Figures, which are described below.

DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 provides a fragmentary and somewhat schematic view of a humanpatient having a continuous cardiac output monitoring catheter instilledinto the pulmonary artery, and a modular monitoring apparatus associatedwith this catheter.

FIG. 2 schematically depicts the functional components of the presentcontinuous cardiac output monitoring apparatus in association with asection of a cardiac output monitoring catheter which is instilled intoa patient's pulmonary artery, and illustrates the functionalrelationship of these components.

FIG. 3 is a schematic diagram illustrating the relationship of thefunctional components of the present continuous cardiac capacitymonitoring apparatus.

FIG. 4 presents a schematic and functional block diagram of thecomponents and interconnections of these components for a poweramplifier embodying the present invention.

FIGS. 5a-5c present voltage wave forms at selected locations in thepower amplifier seen in FIG. 4, with these wave forms set out on acommon time scale.

FIG. 5d presents a resulting voltage wave form resulting from the waveforms seen in FIGS. 5a-5c, and with the time scale compressed to allowdepiction of several voltage wave form cycles.

FIGS. 5e and 5f show further resulting voltage wave forms which areshown on a slightly expanded time scale and with an expanded voltageamplitude scale as well.

FIGS. 6-11 provide fragmentary views of a power amplifier electricalcircuit embodying the present invention.

DETAILED DESCRIPTION OF AN EXEMPLARY PREFERRED EMBODIMENT OF THEINVENTION

FIG. 1 shows a human patient 10, who, for example, may have suffered acoronary occlusion or heart attack, or who may have suffered severetrauma such as may occur from a motor vehicle accident. For the heartattack patient, because damage to the patient's heart 12 occurs not onlyas a result of an initial occlusion, or blockage of a coronary artery,but also over a period of several hours or several days as portions ofthe patient's heart become necrotic as a result of the occluded coronaryblood supply, the patient's coronary capacity is at risk of failure overthis considerable time period following the heart attack. For the traumapatient, who may have lost a considerable quantity of blood, and who maybe in shock, dilation of peripheral vasculation of the body may resultin a decrease in blood pressure while the coronary capacity of thepatient is decreased over a period of time. During these time periods,early detection of an impending heart failure is very important so thatearly interventional measures can be taken while these measures can havetheir best effect.

Experience has shown that monitoring of the patient's blood pressurealone, or monitoring of blood oxygen saturation levels at theextremities of the patient 10, for example, are not adequate indicationsof impending heart failure. As a result, conventional technology hasbeen developed which effects a monitoring of the patient's pulmonaryblood circulation, as well as other related factors, such as oxygensaturation of the blood flowing to the patient's lungs, directly at thepulmonary artery 14 of the patient 10.

This monitoring of pulmonary blood flow is effected by instilling amonitoring catheter 16 into the patient's right jugular vein. A distalend portion 18 of this catheter is advanced down the vein into the rightatrium 20 of the heart 12. From the right atrium 20, the distal endportion 18 is advanced through the tricuspid valve 22 and into the rightventricle 24 of the heart 12. Subsequently, the distal end portion 18 ofthe catheter 16 is advanced through the pulmonary valve 26 and into thepulmonary artery 14. As those who are ordinarily skilled in thepertinent arts will appreciate, usually, an inflatable balloon portion28 of the catheter 16 will be inflated for this introduction procedureso that the prevailing blood flow helps in moving or floating thecatheter along to its desired location.

Externally of the patient 10, the catheter 16 is connected at aplug-and-socket interface 16'/30' to a multi-conductor electrical cable30. This cable 30 provides similar plug-and-socket connection toelectrical circuitry located at a general purpose variably-configurablemodular-type of monitor console 32. This monitoring console 32 includesa display screen 34 upon which information about the patient's conditioncan be locally displayed. Also, this monitor 32 includes a data outputfacility, such as a computer system RS-232 port (schematically indicatedwith the arrowed numeral 36), and by which patient information isprovided to a remote location, such as to a nurse's station, centralpatient monitoring and data recording computer system, or to a physicianwho may wish to receive the information at his home or office via atelephone line interconnection with such a hospital's central patientmonitoring computer system.

The console 32 includes a plurality of electrical interconnectionapertures or ports 38 into which configuration modules may be receivedin order to configure the monitor to perform those monitoring functionswhich are required by particular patients. In the present case, theconsole 32 has received a module 40 for monitoring cardiac output of thepatient 10. The cable 30 has direct plug-and socket connection to themodule 40, which provides interface between the catheter 16 and theconsole 32. The module 40 includes a second cable connector 41 which isconfigured to receive the connector 30' at the distal end of the cable30 where catheter 16 connects for monitoring of the patient 10. As willbe seen, the module 40 can also be used to verify the correct operationof the cable 30 by connection of both ends of the cable 30 to the module40.

With attention now to FIG. 2, it is seen that the module 40 hasconnection to the monitor 32 via a data bus, generally indicated by thedouble-headed arrow 42, and via a number of electrical connections whichsupply electrical power to the module 40 from the console 32. It will beappreciated that FIG. 2 is very schematic, and that the electricalconductors depicted outwardly of catheter 16 are actually of a finegauge and over a portion of their length are disposed within theelongate and comparatively thin shaft of the catheter. Another portionof the length of the illustrated conductors will be understood to beprovided by the cable 30. Two of the conductors 46 and 48 connect to aresistance heating element 50 which is outwardly disposed on the distalend portion 18 of the catheter 16. The heating element 50 may actuallybe configured as a flexible thin metallic film element having a highcoefficient of resistance change with change in temperature.

The catheter 16 will preferably be configured so that this heatingelement 50 is actually disposed in the right ventricle of the patient10. The turbulent blood flow in this ventricle resulting from thepumping action of the heart assists in distributing heat energy from theheating element 50 uniformly throughout the pulmonary blood flow.Downstream of the heating element 50 with respect to the direction ofblood flow (indicated with arrows 52) is disposed a temperaturemeasuring sensor 54. The sensor 54 may be a small bead thermistor, forexample, and is connected to the cable 30 and console 32 via conductors56 and 58. Within the module 40, the conductors 56 and 58 supply thetemperature signal from sensor 54 to a microprocessor based controlsystem 56, including a microprocessor 58 and power amplifier circuit 60.The microprocessor 58 has a two-way control and data interface with thepower amplifier circuit 60, as is generally indicated by the control anddata bus arrow 62. This general interface reference numeral (62) is usedthroughout the following explanation to refer to the interface ofinformation and control signals in one or both directions between thepower amplifier 60 and the microprocessor 58.

FIG. 3 shows that the power amplifier 60 includes a programmableselectively variable voltage source section 64, a frequency sourcesection 66, and a switch-mode amplifier section 68 which receives asinputs both electrical power at a selected programmable voltage levelfrom the section 64, and a reference frequency signal from the section66, and which combines these inputs to provide frequency-controlledalternating current electrical power to an isolated patient-connectedsection 70 with a pure sine wave form at a selected and variable voltagelevel. Electrically, the patient-connected section 70 is defined in partby the catheter 16 and the cable 30. The programmable selectivelyvariable voltage source 64 has connection, as is depicted at 72, to a 28volt direct current power source (not shown). A power cut off relay 74is under the control of the microprocessor 58, as is indicated by theinterface connection 62. As will be seen, this control of relay 74 bymicroprocessor 58 is redundant, and is further backed up by control ofthe voltage source 80 by the microprocessor 58 so that the relay 74 canbe opened or voltage source 80 may be commanded to provide a zero outputvoltage, all in order to safeguard the patient 10 from inadvertentinjury by excessive heating at catheter 16. This relay 74 supplieselectrical power to a pair of semiconductor switches 76 and 78. Switches76 and 78 are controlled by a voltage regulator circuit 80, which isalso under the control of the microprocessor 58, as is indicated by theinterface connections 62.

As will be further explained below, the applicants have adapted aconventional semiconductor integrated circuit voltage regulator, whichis designed to provide a steady regulated voltage output level even ifits supply voltage varies, and have created a programmable selectivelyvariable direct current voltage supply. In the present case, theprogrammable voltage supply has a resolution of 2¹², or 4,096 differentincremental voltage levels which may be individually selected by themicroprocessor 58 in order to control the level of resistance heatingand energy dissipation at the heating element 50 of catheter 16.Accordingly, the level of electrical power which is supplied by themodule 40 to the heating element 50 of the catheter 16 is under veryfine control by the microprocessor 58.

Direct current electrical power of finely controlled voltage level issupplied by the voltage source section 64 to the amplifier section 68,as is indicated by the schematic conductor 82. The power amplifiersection 68 also receives a precisely regulated frequency signal from thefrequency source section 66, as is indicated by the schematic conductorconnections 84 and 86. As can be seen from the schematic illustration ofFIG. 3, the frequency source section 66 includes a 1 MHz crystalreference oscillator 88. This oscillator 88 provides an asymmetrical(i.e., positive-going only) output signal at a precise 1 Mhz rate with ahigh-signal duty cycle of about 40 percent. This signal is provided to adivider circuit 90, which first effects a division by 5 to provide a 200KHz signal, which is still only positive going, and has a low dutycycle. As an aid to the reader at this point in the circuit description,small graphical signal wave-form illustrations have been added to FIG.3.

Next, the divider circuit 90 effects a division by 2 to provide a 100KHz signal which toggles between a signal-high value and signal-low(zero) value 100 thousand times a second. While this signal is a squarewave form and is only positive-going, it is the basis for a puresymmetrical alternating current wave form which will be provided by thepower amplifier 60. As those who are ordinarily skilled in the pertinentart will be aware, a pure square wave form of 50 percent duty cycle hasonly odd-factored harmonics added to a pure sine wave form when analyzedby Fourier analysis. As will be seen, the wave form provided by thedivider 90 is used to effect a switching of direct current into a puresquare wave form from which the odd-factored harmonics are removed, toprovide a pure sine wave form of alternating current output from thepower amplifier 60. In order to prevent simultaneous conduction, and aresulting short circuit, during switching of the direct current powerfrom the voltage source section 64, a dead-time generator is provided toprovide a pair of oppositely-going square wave signals which still havea 50 percent duty cycle. These signals are provided to respective onesof a pair (94, 96) of switch drivers. These switch drivers 94, 96 inturn control switching of respective ones of a pair of semiconductor(MosFet) switches 98, 100, which are part of the power amplifier section68.

Considering now the power amplifier section 68, it is seen that theswitches 98, 100 control current flow through opposite sides of acenter-tapped transformer 102. Preferably, this transformer has a turnsratio of substantially 1:1.6 As mentioned above, this center tappedtransformer 102 provides a square wave output at a frequency of 100 KHzwith a 50 percent duty cycle. Consequently, this square wave has thecharacteristics of a pure sine wave with only odd-numbered harmonicfactors added. Power amplifier section 68 includes first and secondseries tuned circuits 104 and 106, which are respectively tuned topresent a very high impedance to the third and fifth harmonic factors ofthe 100 KHz signal from transformer 104. That is, circuit 104 is tunedto present a very high impedance so as to effectively block the 300 KHzcomponent of the signal from transformer 102, while the circuit 106 istuned to block the 500 KHz component. This blockage of the third andfifth harmonic components of the selected frequency signal from thetransformer 102 is an important aspect of the present power amplifierbecause these first two odd-numbered (third and fifth) harmonics carrythe most energy. By presenting a high impedance at the circuits 104 and106, a large portion of this energy of the third and fifth harmonicswill be reflected and will not be lost to inefficiency in the poweramplifier 60.

Next, the power amplifier section 68 includes a series tuned circuit108, which presents a very low impedance to the 100 KHz selectedfrequency, while presenting a high impedance to higher order harmonicfrequencies of the 100 KHz selected frequency. The remaining portions ofthe higher order harmonics which pass the tuned circuit 108 are shuntedto ground by a high-order shunt tuner circuit 110. This shunt tunercircuit 110 drives a resulting sine wave voltage signal into the primarywinding of an isolation transformer 112 having a turns ratio ofsubstantially 3:1. As is seen on FIG. 3, a voltage sensing circuit 114is associated with the connection between the shunt tuner circuit 110and the primary winding of the transformer 112 in order to provide afeed back value of voltage supplied to the catheter 16 to themicroprocessor 58, as is indicated by the interface arrow 62.

Importantly, the ground side of the transformer 112 is connected to theground indicated at 116 via a second 100 KHz series tuner circuit 118.The series tuned circuits 108 and 118 are separated from one another bythe high reflected impedance appearing at the primary winding of theisolation transformer 112 by virtue of its 3:1 turns ratio so that theinductances and capacitances of these tuners do not simply add with oneanother to result in a composite tuned circuit which would have a tunedfrequency other than 100 KHz. Accordingly, the tuned circuits 108 and118 can each participate in insuring that substantially only a pure sinewave voltage form at 100 KHz is effective at the primary winding oftransformer 112. FIG. 3 also shows that the power amplifier section 68of power amplifier 60 has connection to the microprocessor 58 via asense common circuit 120, and a current flow sensing circuit 122.

The isolated patient-connected circuit section 70 effectively floatswith respect to ground potential because no ground connection is madeacross the isolation transformer 112. This isolated patient-connectedsection 70 of the power amplifier circuit 60 includes the resistiveheating element 50 of the catheter 16, and a calibration resistor 124.Each of these resistances has a value of substantially thirty nine ohms,so that the reflected impedance at the primary winding of the isolationtransformer is substantially 350 ohms (i.e., 39 ohms multiplied by thesquare of the turns ratio across transformer 112). Also, isolatedpatient-connected circuit section 70 includes a relay 126 switching theconnection of the secondary winding of transformer 112 between theheating resistor 50 and the calibration resistor 124. This switchingrelay 126 has a connection with the microprocessor 58, as is indicatedwith interface arrow 62. Accordingly, the microprocessor 58 can not onlycontrol the condition of relay 126, but can also verify this condition(as will be further explained), so that the microprocessor 58 can verifythat electrical power is applied to the heating resistor 50 only whencommanded by the microprocessor 58, and cannot be applied otherwisewithout the microprocessor taking positive action to shut down theentire module 40.

Considering now FIG. 4, a more greatly detailed presentation of thefrequency source circuit section 66 is presented. This circuit sectionincludes the crystal oscillator 88, which provides the square wavepositive-going signal seen in FIG. 5A to the divider 90 via a conductor128. Divider 90 effects first a division by five to produce the signalseen at FIG. 5B. This signal varies between zero and a positive valueupon each fifth positive-going signal transition of the signal seen inFIG. 5A. The value of the signal seen in FIG. 5B drops back to zeroafter a comparatively short time interval, which is considerably shorterthan the time required for five cycles of the signal of FIG. 5A to pass.Importantly, the signals seen in FIGS. 5A and 5B are not square waveswith a 50 percent duty cycle, and will not satisfy the relationshipexplained above with respect to having only odd-ordered harmonics withrespect to a pure sine wave signal. The signal seen in FIG. 5B is outputby circuit 90 on a conductor 130, which returns this signal to thecircuit 90 to a terminal at which a division by two is effected. Thedivision by two results in a signal seen in FIG. 5C which togglesbetween zero and a positive value at a rate of 100 KHz. This signal is asquare wave with a 50 percent duty cycle, but is positive-going only.That is, the signal of FIG. 5C is not symmetrical about the zero voltageaxis.

This signal of FIG. 5C is provided by a conductor 132 to the dead timegenerator circuit 92. This circuit 92 includes a pair ofoppositely-connected exclusive-OR (XOR) gates 134, and 136. The gate 134is connected at one input terminal to the positive input V_(CC)(indicated with the numeral 138), and at the other input terminalreceives the signal of FIG. 5C.

Consequently, the gate 134 conducts only while the signal of FIG. 5C ispositive. On the other hand, the gate 136 is connected at one inputterminal to the signal of FIG. 5C, and at the other input terminal isconnected to ground (indicated with the numeral 140). This gate 136consequently conducts only while the signal of FIG. 5C is zero. Theresult is that the gates 134 and 136 each provide respective ones of apair of time-matched square-wave signals which are positive-going only,and are oppositely time sequenced, as is seen in FIG. 5D. The readerwill note that the time scale of FIG. 5D is considerably compressed incomparison to that of FIGS. 5A through 5C, so that several cycles of thetime-matched oppositely-sequenced signals can be shown. These signals ofFIG. 5D are provided on respective conductors 142 and 144 to respectiveresistor-capacitor networks 146 and 148, each one of which also includesa respectively oriented diode conducting toward ground potential so thatthe signal provided by the gates 134 and 136 can transition high tocharge the capacitor (as seen on a respective conductor 150 and 152),but can transition low only with the added effect of theresistor-capacitor time constant resulting from the networks 146 and148. The resulting signals at the conductors 150 and 152 are seen inFIG. 5E. These signals are still only positive-going, and between thetwo signals have a duty cycle of 50 percent.

The signals of FIG. 5E are provided to a pair of exclusive-OR gates 154and 156, which are connected together at one of their input connectionsby a conductor 158. The conductor 158 has a ground connection, indicatedby numeral 160. Consequently, the gates 154 and 156 each individuallyonly conduct when the signal received from the conductors 150 and 152(the signal of FIG. 5E) are high. However, these gates do not switch offas soon as the signal of FIG. 5E drops below it greatest high value.Instead, these gates switch off at some voltage level intermediate ofthe high and low (zero) signal levels. Because of the resistor-conductortime constants effective on the zero-going portion of the signals seenin FIG. 5E, the gates 154 and 156 do not switch off simultaneously withthe signals seen in FIG. 5D, but have their switching off delayed untilsome lower but non-zero voltage value, indicated with the numeral 162 isreached. As a result, the gates 154 and 156 provide on respectiveconductors 164 and 166 respective signals as are indicated in FIG. 5F.These signals are still only positive-going, and have a 50 percent dutycycle between the two of these signals. However, the switching-offtransition (i.e., the negative going part of the square wave form) ofeach wave form is delayed slightly with respect to the positive-goingtransition of the companion wave form seen in FIG. 5F. Conductors 164and 166 provide the signals seen in FIG. 5F to a switch driver circuit168, which inverts each of these signals so that the positive-goingturn-on part of the signal is delayed with respect the negative-goingturn-off portion of the signal. The switch driver circuit 168 providesrespective inverted signals of the same wave form as is seen in FIG. 5F,but of respectively inverted shape and transposed time sequencing, onconductors indicated with the numerals 170 and 172. The use made ofthese signals with be further explained below.

Viewing now FIGS. 6-8 in conjunction with one another, the voltageregulator 80 and switches 76, 78 of the programmable selectivelyvariable voltage source circuit 64 of FIG. 3 is shown in greater detail.Particularly, FIG. 6 schematically depicts the overall circuit schematicfor a regulated voltage supply using a voltage regulator integratedcircuit 174. This circuit 174 provides an output voltage on a conductor175 which is equal to a constant multiplied by the sum of one plus thequotient of the value of resistor R₁ divided by the value of resistorR_(x). According to the present preferred embodiment of the invention,this constant is 1.25. As is seen in FIG. 6, resistor R_(x) is avariable resistor according the present invention. Further, as will beseen, the present invention includes provision for the value of resistorR_(x) to be digitally programmable and to be controlled bymicroprocessor 58. The value of resistor R_(x) is programmable with aresolution of 2¹² incremental values. Accordingly, it will be seen thatthe regulated and controlled voltage level provided by the voltageregulator circuit 174 is controllable by the microprocessor 58 with afine degree of control.

Turning now to FIGS. 7 and 8 in combination, it is seen that the voltageregulator circuit 80 includes a resistor designated R₁, which is seen inFIG. 7, and which functions as the resistor designated in the same wayand seen in FIG. 6. The voltage regulator circuit and switches 76 and 78are seen in FIG. 8. Also, this voltage regulator circuit 174 sectionincludes an array of four digitally-controlled analog switches 176, 178,180, and 182, which are seen in FIG. 7. The analog switches 176-182 areeach controlled by the microprocessor 58 via the interface connectionsdesignated with the numeral 62, and seen along the left side of FIG. 7.That is, the microprocessor 58 can drive up the signal level atindividual ones or groups of as many as all of the signal leads to theseanalog switches. A signal-high value on any one of the conductorsindicated results in the associated switch 176-182 switching arespective one of the resistors indicated below into a parallelresistance relationship to ground.

To further explain the above, it is seen that the analog switches176-182 also have connection individually with an array of twelveresistors, which are designated R₂ through R₁₃, and which areindividually switched into connection with a grounded conductor 186 whenthe respective one of the leads indicated with the interface numeral 62are driven signal-high by the microprocessor 58. These resistors R₂through R₁₃ collectively function as the variable resistor R_(x) seen inFIG. 6. These resistors have increasing values ranging generally fromabout 200 ohms to about 422 Kohms. Particularly, the resistors R₂through R₁₃ have values in ohms of: 200, 402, 806, 1.62K, 3.24K, 6.49K,13.0K, 26.1K, 52.3K, 105K, 210K, and 422K. An example of an analogswitch which has proved to be acceptable for use as the switches 176 and178 for switching the resistors having the lower values (i.e., in therange from about 200 ohms to about 1.6 Kohms), is the Siliconix 9956DY.This analog switch has a very low resistance when switched on. Thus, theresistance of the switches 176 and 178 themselves does not itself addappreciable to the resistances of the resistors R₂ through R₅. On theother hand, for the analog switches 180 and 182, a Harris DG412DY hasshown to be acceptable. This analog switch has a very low leakagecurrent when switched off so that the comparatively small incrementalchange in current flow which results when the higher-valued resistors R₆through R₁₃ are switched in parallel into the circuit can be easilydistinguished from the leakage current through the switches 180 and 182themselves. The resistors R₆ through R₁₃ have values greater than 1.6Kohm, up to or greater than about 422 Kohm. Preferably, the resistors R₂through R₁₃ are precision 0.1 percent, 50 PPM resistors in order toenable closer calibration of the voltage supplied by the voltageregulator 174.

It will be noted viewing FIG. 7 that a resistor R₁₄ is provided toadjust the maximum resistance value appearing on conductor 186 alongwith a resistor R₁₅ and a trimming resistor R₁₆ adjusting the effectiveresistance appearing at conductor 192. The value of the resistor R₁₅sets the maximum voltage which the voltage regulator 174 will supplyeven when all of the resistors R₂ through R₁₃ are switched to ground byclosing of all of the analog switches 176-182. The conductor 192 appearsat the left side of FIG. 8, and has connection with the voltageregulator circuit 174. This conductor 192 is analogous to the conductorschematically seen at the upper end of resistor R_(x) of FIG. 6, andhaving connection with the regulator circuit 174. The effectiveresistance value to ground from conductor 192 controls the voltage leveloutput by voltage regulator circuit 174. As is seen, the microprocessor58 controls this effective voltage level by effecting switching ofresistors R₂ through R₁₃ into connection with grounded conductor 186 viathe analog switches 176-182. Also, the microprocessor 58 can control thevoltage regulator circuit 174 through the indicated interface line 62 sothat the voltage regulator 174 is turned off to provide no output power,or is turned on to provide output power of the voltage level selected bythe switched condition of the analog switches 176-182. Accordingly, themicroprocessor 58 can control the power level of electrical heatingeffected at the heating element 50 of the cathode 16 from zero to thefull wattage capacity of this heater.

Viewing FIG. 8, another feature of the present voltage source circuitsection will be seen. An example of an integrated circuit acceptable foruse as the voltage regulator 174 is the Linear Technology LTC1149. Thisvoltage regulator has a constant off time architecture, rather than afixed switching frequency. Consequently, operating frequency of thisvoltage regulator will vary with output voltage. For the presentapplication, the output voltage can vary between 1 volt and 26 volts DC.Recalling FIG. 6 it is seen that the voltage regulator circuit 174requires a capacitance to ground connected in common with the conductorconnecting to the reference resistor R₁. In order to avoid the use oflarge high-voltage capacitors and preserve the small size, weight, andcost goals of the present power amplifier, this capacitance is providedby a capacitor array indicated with the numeral 194 on FIG. 8. Thecapacitor array includes a plurality of capacitors connected between theconductor 82 (which is the regulated-voltage power output conductor forthe selectively-variable voltage source circuit section 64, recallingthe description of FIG. 3), and ground. The capacitor array 194 includesa number of equally valued resistors 196, which serve as voltage sharingresistors among the capacitors of the array 194, distributing thevoltage drop equally across these capacitors and preventing an excessivecurrent flow in any one of the capacitors.

Viewing now FIG. 9, the power amplifier circuit section 68 and isolatedpatient-connected circuit section 70 are shown in greater detail.Recalling the description of the power amplifier circuit section 68, itis seen that this circuit section receives the signals of FIG. 5F onconductors 170 and 172. This circuit section also receives theselectively varied voltage output from voltage source circuit section 80on conductor 82. The signals on conductors 170 and 172 drive MosFetswitches 98 and 100 alternately into conduction, with the indicated deadtime preventing simultaneous conduction of through these switches, sothat the creation of a short from conductor 82 to ground connection 198is prevented. The alternate current conduction through the switches 98and 100 drives the center-tapped transformer 102 to provide anessentially symmetrical square wave output of 50 percent duty cycle intothe first 300 KHz trap 104. This trap 104 includes a capacitor 200 andinductor 202 which in combination are tuned to present a high impedanceto a 300 KHz frequency. Similarly, the 500 KHz trap 106 includes acapacitor 204 and inductor 206 which in combination are tuned to providea high impedance to a 500 KHz frequency. The 100 KHz series tuner 108 ofFIG. 3, is formed by the interaction of the two inductors 104 and 106 inseries with a capacitor 208. These components are tuned to present a lowimpedance to a 100 KHz frequency and a comparatively high impedance tohigher order (i.e., the 7th, 9th, etc.) harmonics of the 100 KHzselected frequency). The shunt tuner 110 of FIG. 3 is actually formed bya capacitor 210, and a pair of parallel connected inductors 212, 214,which pass higher order harmonics (now of comparatively low energylevel) to ground connection 116. Series tuner 118 is formed by thecooperation of a capacitor 218 and an inductor 220, allowing theselected 100 KHz frequency to reach ground 116 with little impedance.Accordingly, the primary winding of isolation transformer 112 receivesessentially alternating current power of essentially pure sine wavecharacteristic. It will be noted that the voltage drop occurring acrossa resistor 222, which is exposed essentially only to the selected 100KHz frequency of the electrical power delivered into the resistive load50 is available to the microprocessor 58 via the interface connections62 bridging this resistor. Accordingly, the microprocessor 58 can verifywhen and if electrical power of the selected frequency is beingdelivered to heater 50 of catheter 16.

In order to further understand the control and safeguard features of thepresent invention, it should be noted that at the isolation transformer112, is established a virtual isolation barrier, indicated with thedashed line 224. To the right hand side of the barrier 224 is theisolated patient connected portions of the power amplifier circuit 60,module 40, and catheter 16, with heater 50. No physical electricalconnection is effected across the barrier 224. In order to control therelay 126, an additional isolation transformer 226 is provided. Thisisolation transformer is powered by a 100 KHz power supply circuit 228seen on FIG. 4. Viewing the power supply circuit 228 on FIG. 4, it isseen that a divider 230 receives the signal of FIG. 5A from theoscillator 88, and is connected just like the divider 90 to provide asignal like that illustrated by FIG. 5C to a transistor 232. Thetransistor 232 toggles on and off in response to the signal from thedivider 230, and similarly causes a second transistor 234 to toggle onand off. This second transistor 234 drives a pair of oppositelyconnected PNP (236) and NPN (238) transistors. The transistors 236 and238 toggle on and off in opposition to one another to provide alow-power 100 KHz alternating current power supply at conductor 240.

Returning to FIG. 9, it is seen that the conductor 240 delivers thislow-power 100 KHz alternating current power to the primary winding ofthe transformer. Thus, the same frequency of alternating current powerwhich has been determined to offer the greatest level of patient safetyis used to effect control of the isolated patient-connected circuitsection 70. The secondary winding of transformer 226 drives a rectifier242 providing direct current power on the isolated patient-connectedcircuit section 70. In order to control the relay 126, themicroprocessor 58 can command illumination of a light emitting diode244. Light from this LED (arrow 246) crosses the barrier 224, and causesa photodiode 248 to become conductive. The photodiode 248 controlscurrent flow through the coil 250 of the relay 126 so that this relay isunder the control of the microprocessor 58 with no physical connectionacross the barrier 224.

In order to inform the microprocessor 58 that power is being dissipatedon the isolated patient-connected side of the barrier 224, the voltagedrop occurring across the calibration resistor 124 is used to drive atransistor switching circuit into conductivity. It will be noted thatthe calibration resistor 124 is in fact formed by two resistorsconnected in parallel. Conductivity at switching circuit 252 illuminatesa LED 254. Again, light from the LED 254 is beamed across the barrier224 (arrow 256), and causes a photodiode 258 to become conductive.Conductivity of the photodiode 258 pulls low the signal on conductor260, which has connection with the microprocessor 58, as is indicated byinterface arrow 62.

Returning to a consideration of FIG. 3, it is seen that themicroprocessor 58 has control over the relay 74, from which power isreceived to operate the entire power amplifier 60. In the event that themicroprocessor 58 is informed that electrical power is being dissipatedinto the isolated patient-connected section 70 when this powerdissipation has not been commanded, then the relay 74 will be opened toshut down the power amplifier. On the other hand, if after commandingthe voltage source 80 (regulator 174) to provide a selected level ofvoltage and the closing of the relay 126, the microprocessor is notinformed within a selected time interval (only a fraction of a second)that power is being dissipated in the isolated patient-connected section70, then a fault is assumed. In this event also, the relay 74 will beopened, or the voltage regulator 174 is alternatively commanded toprovide a zero voltage output, and the patient is protected from any andall inadvertent injury which might result from operation of the heatingelement 50 in the pulmonary artery 14 without adequate control.

In order to provide the desired degree of safeguarding over unintendedor inadvertent operation of the heating element 50, the power amplifiercircuit 60 includes the circuit sections 262 and 264, which areillustrated in FIGS. 10 and 11, respectively. Particularly viewing thecircuit portion 262, it is seen that the conductor 266 is seen in FIG. 7to be connected to the regulated voltage output of the voltage source174 at conductor 82 via a pair of voltage dividing resistors indicatedwith the numeral 268. The voltage level appearing at the conductor 266is an indication of the voltage level actually provided by the voltageregulator 174. This voltage level is provided to a unitary gain buffer270. This buffer 270 is an operational amplifier which provides anoutput to the microprocessor 58, indicated with the general interfacenumeral 62. Accordingly, the microprocessor 58 can read the voltagelevel provided by the voltage source 80 using voltage regulator 174. Inthe event that a fault in the regulator 174 or some other portion of thepower amplifier 60 causes a voltage other than an acceptable andexpected value to appear from the buffer 270, then the microprocessor 58will effect a shutdown of the power amplifier 60.

Turning now to FIG. 11, the circuit section 264, which effects controlof the relay 74 (recalling FIG. 3), is depicted. The circuit section 264includes the relay 74 itself, and a transistorized switching circuitindicated with the numeral 272. This transistorized switching circuit272 places the relay 74 under the control of the microprocessor via aninterface effected by conductor 274. That is, a signal-high value at theconductor 274 provided by the microprocessor 58 will result in theswitch circuit 272 closing and in the closing of the relay 74 to powerthe power amplifier 60. However, this closing of the switching circuit272 can be effected only so long as the validity of this action isverified by an internal watch dog timer (not shown), which is associatedwith the microprocessor 58. In other words, if an internally repeateddiagnostic of the microprocessor 58 is not successfully completed, thewatch dog will reset and reboot the microprocessor 58 and will effect ashut down of the power amplifier 60 by pulling the signal on conductor274 signal-low. This pull down of the signal on conductor 274 iseffected through the diode 276, which has connection to the watch dogportion of the microprocessor 58 as is indicated by the interfacenumeral 62.

Turning once again to FIG. 1, it will be recalled that the module 40includes provision to verify the correct functioning of the cable 30.That is, the cable 30 could be damaged during use of the monitoringapparatus including console 32, catheter 16, module 40 and cable 30. Theconsole 32 and module 40 are durable components, while the catheter 16is a single use apparatus. Consequently, a new catheter 16 is used witheach patient. However, the cable 30, although it is a durable componentis by far the most subject to damage from rough use, or by being steppedon, for example, in the medical treatment use environment for theapparatus described. In order to test and verify correct operation ofthe cable 30, the connector portion attached to the cable 30 at theplug-and-socket connection 16'/30' is connected back into the module 40at the connector 41 provided on this module 40.

FIG. 9 illustrates with dashed lines 278 the electrical configurationeffected by this connection of the cable 30 back into connector 41. Inother words, with the cable 30 connected into connector 41, theresistance heating element 50 of a catheter 16 is not connected to theoutput of the relay 126. However, the calibration resistors 124 arestill connected at the contacts of this relay 126 to which theyordinarily connect, and are also now connected via cable 30 to thecontacts of this relay at which the resistance heating element 50 of thecatheter 16 is connected in the use configuration of the apparatus.Consequently, with the cable 30 connected to the connector 41, if theconsole 32 is used to effect a calibration of the module 40 and catheter16, the module 40 will read the calibration resistors 124 in thecalibration sequence, and then will read these calibration resistors 124again via the cable 30 as though it were testing the operationalreadiness of a catheter. In this test sequence, if there is more than apredetermined difference in resistance between the calibration resistors124 and the heating resistor 50 of a catheter, the module 40 will signalvia console 32 that the catheter is bad. However, in the cable testconfiguration described, this "bad catheter" signal will mean that thecable 30 itself is defective.

While the present invention has been depicted, described, and is definedby reference to a particularly preferred embodiment of the invention,such reference does not imply a limitation on the invention, and no suchlimitation is to be inferred. The invention is capable of considerablemodification, alteration, and equivalents in form and function, as willoccur to those ordinarily skilled in the pertinent arts. The depictedand described preferred embodiment of the invention is exemplary only,and is not exhaustive of the scope of the invention. Consequently, theinvention is intended to be limited only by the spirit and scope of theappended claims, giving full cognizance to equivalents in all respects.

We claim:
 1. A selectively programmable variable-voltage direct-currentvoltage source for providing an output voltage, the output voltage beingcontrolled by a microprocessor, said voltage source comprising amicroprocessor:a pair of controlling resistors each having a resistancevalue, said controlling resistors including a variable resistor; avoltage regulator connected to said controlling resistors and forproviding the output voltage; and a programmable variable-resistancecircuit including:plural resistors connected in a parallel configurationand having a common connection, said voltage regulator being connectedto said common connection, each resistor of said plural resistors havinga differing resistance value; and switch means connected to themicroprocessor and said plural resistors for receiving instructions fromthe microprocessor and for responsively connecting selected ones of saidplural resistors in parallel with respect to said common connection todefine said variable resistor with a resistance value.
 2. Theselectively programmable variable-voltage direct-current voltage sourceof claim 1 wherein said voltage regulator includes a Linear TechnologyLTC1149 circuit.
 3. The selectively programmable variable-voltagedirect-current voltage source of claim 1 wherein said voltage regulatoris connected to the microprocessor;said voltage regulator beingcontrolled by the microprocessor to provide the output voltage with azero value.
 4. A selectively programmable direct-current voltage sourcefor providing a variable output voltage, said voltage sourcecomprising:a pair of controlling resistors each having a resistancevalue, said controlling resistors including a variable resistor; avoltage regulator connected to said controlling resistors and forproviding the output voltage; and a programmable variable-resistancecircuit including:digital microprocessor means; plural resistorsconnected in a parallel configuration and having a common connection,said voltage regulator being connected to said common connection eachresistor of said plural resistors having a differing resistance value;and switch means connected to said digital microprocessor means and saidplural resistors for receiving instructions from said digitalmicroprocessor means and for responsively connecting selected ones ofsaid plural resistors in parallel with respect to said common connectionto define said variable resistor with a resistance value; said switchmeans including a plurality of digitally controlled analog switches eachone of which has connection to respective ones of said plural resistors,and which analog switches when closed connect respective ones of saidplurality of resistors in parallel between said voltage regulator andsaid common connection; and said digital microprocessor means havingconnection with said analog switches for commanding selective closure ofparticular ones of said analog switches to alter the effective parallelresistance value connected to said voltage regulator as said variableresistor.
 5. The selectively programmable variable-voltagedirect-current voltage source of claim 4 wherein said plurality ofdigitally controlled analog switches includes a switch selected from thegroup including a Siliconix 9956DY and a Harris DG412DY.
 6. Theselectively programmable variable-voltage direct-current voltage sourceof claim 4 wherein said plurality of analog switches includes aSiliconix 9956DY which is connected to resistors of said plurality ofresistors which have resistance values up to about 1.6 Kohm.
 7. Theselectively programmable variable-voltage direct-current voltage sourceof claim 4 wherein said plurality of switches includes a Harris DG412DYwhich is connected to resistors of said plurality of resistors whichhave resistance values of at least about 1.6 Kohm.
 8. A voltage sourcefor providing variable direct-current (DC) voltage under control of amicroprocessor, said voltage source comprising a microprocessor:avariable-resistance circuit including:a common ground; a plurality ofresistors connected in a parallel configuration and to said commonground; and a plurality of switches connected to the microprocessor andto said plurality of resistors for receiving instructions from themicroprocessor and for responsively connecting selected ones of saidplurality of resistors in parallel with respect to said common ground todefine collectively a control resistor with a resistance value; and avoltage regulator circuit connected to said control resistor and havingan output for providing the variable DC voltage, said voltage regulatorcircuit being connected to said control resistor such that the variableDC voltage changes in response to changes in said resistance value ofsaid control resistor.
 9. The voltage source of claim 8 wherein saidvoltage regulator circuit includes a constant resistor with a resistancevalue connected between said plurality of resistors and said output ofsaid voltage regulator circuit;said voltage regulator circuit forproviding the variable DC voltage as a ratio of said resistance valuesof said constant resistor and said control resistor.
 10. The voltagesource of claim 8 wherein each resistor of said plurality of resistorshas a different resistance value.
 11. A voltage source for providingvariable direct-current (DC) voltage under control of a microprocessor,said voltage source comprising a microprocessor:a variable-resistancecircuit including:a common ground; a plurality of resistors connected ina parallel configuration and to said common ground; and a plurality ofswitches connected to the microprocessor and to said plurality ofresistors for receiving instructions from the microprocessor and forresponsively connecting selected ones of said plurality of resistors inparallel with respect to said common ground to define collectively acontrol resistor with a resistance value; and a voltage regulatorcircuit connected to said control resistor and the microprocessor andhaving an output for providing the variable DC voltage, said voltageregulator circuit being connected to said control resistor such that thevariable DC voltage changes in response to changes in said resistancevalue of said control resistor; said voltage regulator circuit beingcontrollable directly by the microprocessor to provide the variable DCvoltage with a zero value.
 12. A voltage source for providing variabledirect-current (DC) voltage under control of a microprocessor, saidvoltage source comprising a microprocessor:a variable-resistance circuitincluding:a common ground; a plurality of resistors connected in aparallel configuration and to said common ground; and a plurality ofswitches connected to the microprocessor and to said plurality ofresistors for receiving instructions from the microprocessor and forresponsively connecting selected ones of said plurality of resistors inparallel with respect to said common ground to define collectively acontrol resistor with a resistance value; a voltage regulator circuitconnected to said control resistor and having an output for providingthe variable DC voltage, said voltage regulator circuit being connectedto said control resistor such that the variable DC voltage changes inresponse to changes in said resistance value of said control resistor;and a capacitor array including a plurality of capacitors connectedbetween said output of said voltage regulator circuit and ground. 13.The voltage source of claim 12 wherein said capacitor array furtherincludes a plurality of resistors connected with said plurality ofcapacitors so as to prevent and excessive current flow in any one ofsaid plurality of capacitors.
 14. A voltage source for providingvariable direct-current (DC) voltage under control of a microprocessor,said voltage source comprising a microprocessor:a variable-resistancecircuit including:a common ground; a plurality of resistors connected ina parallel configuration and to said common ground; and a plurality ofswitches connected to the microprocessor and to said plurality ofresistors for receiving instructions from the microprocessor and forresponsively connecting selected ones of said plurality of resistors inparallel with respect to said common ground to define collectively acontrol resistor with a resistance value; a voltage regulator circuitconnected to said control resistor and having an output for providingthe variable DC voltage, said voltage regulator circuit being connectedto said control resistor such that the variable DC voltage changes inresponse to changes in said resistance value of said control resistor;and a power cut-off relay connected to said voltage regulator circuitand the microprocessor; said power cut-off relay being controllable bythe microprocessor to cause said voltage regulator circuit to providethe variable DC voltage with a zero value.